Recording medium, ink-jet recording method using the same, and dispersion of alumina hydrate

Abstract: Disclosed herein is a recording medium comprising an alumina hydrate containing 0.01 to 1.00 wt.-% of titanium dioxide. This recording medium is used in an ink-jet recording method.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a recording medium suitable for use in recording using water-based inks, ink-jet recording method and a dispersion. In particular, this invention relates to a recording medium which can provide images high in optical density, bright in color tone and high in resolution, and has excellent ink-absorbing capacity, an ink-jet recording method using such a recording medium, and a dispersion of an alumina hydrate, which is suitable for use in production of the recording medium. Related Background Art In recent years, an ink-jet recording system, in which minute droplets of an ink are flown by any of various working principles to apply them to a recording medium such as paper, thereby conducting recording of images, characters and/or the like, has been quickly spread as a recording apparatus for various images in various applications including information instruments because it has features that recording can be conducted at high speed and with a low noise, color images can be formed with ease, recording patterns are very flexible, and development and fixing process are unnecessary. Further, it begins to be applied to a field of recording of full-color images because images formed by an multi-color ink-jet recording system are comparable in quality with multi-color prints by a plate making system and photoprints by a color photographic system, and such records can be obtained at lower cost than the usual multi-color prints and photoprints when the number of copies is small. With the improvement in recordability, such as speeding up and high definition of recording, and multi-coloring of images, recording apparatus and recording methods have been improved, and recording media have also been required to have higher properties. In order to satisfy such requirements, a wide variety of recording media have been proposed. For example, Japanese Patent Application Laid-Open No. 52-53012 discloses paper for ink-jet, in which a base paper web low in sizing degree is impregnated with a surface coating. Japanese Patent Application Laid-Open No. 53-49113 discloses paper for ink-jet, in which a sheet containing urea-formalin resin powder therein is impregnated with a water-soluble polymer. Japanese Patent Application Laid-Open No. 55-5830 discloses paper for ink-jet recording, in which a coating layer having good ink absorptiveness is provided on a surface of a base. Japanese Patent Application Laid-Open No. 55-51583 discloses that amorphous silica is used as a pigment in a coating layer. Japanese Patent Application Laid-Open No. 55-146786 discloses that a coating layer formed of a water-soluble polymer is used. In U.S. Patent Nos. 4,879,166 and 5,104,730, and Japanese Patent Application Laid-Open Nos. 2-276670, 4-37576 and 5-32037, there have been proposed recording sheets having a layer using an alumina hydrate of a pseudoboehmite structure. As described in U.S. Patent Nos. 4,379,804 and 5,104,730, and Japanese Patent Application Laid-Open Nos. 58-110287 and 4-37576, it has also been conducted to form an ink-receiving layer of a multi-layer structure using a silica or alumina material. However, the conventional recording media have involved the following problems: 1) U.S. Patent No. 5,104,730, and Japanese Patent Application Laid-Open Nos. 2-276670, 2-276671 and 3-275378 disclose recording media having extremely narrow distribution of pore radius. As disclosed in Japanese Patent Application Laid-Open Nos. 4-267180 and 5-16517, however, individual dyes for inks (cyan, magenta, yellow and black) and solvents for the inks are selectively adsorbed in pores of a specific size. Therefore, bleeding may occur on prints if the composition of ink is changed. The bleeding refers to a phenomenon that edges of boundaries of multi-color printed areas can not be resolved because of feathering caused when ink is still fluid before it is fixed in the ink-receiving layer. 2) U.S. Patent No. 5,104,730, and Japanese Patent Application Laid-Open Nos. 2-276670, 2-276671 and 3-275378 disclose recording media having pore radius distribution as narrow as 1 to 3 nm (10 to 30 Å) in average pore radius. In this pore radius distribution, dye adsorptiveness is good, while the absorptiveness of solvent is insufficient, resulting in the occurrence of beading. The beading mentioned in the present invention refers to a phenomenon in which dots irregularly move in the plane direction of the surface of an ink-receiving layer when ink is still fluid before it is fixed in the ink-receiving layer, thus forming new aggregates together with adjacent dots to cause an unevenness in the density of recorded images. 3) In printing of color images, inks increase in quantity. The inks printed cannot be completely absorbed in pores, but run out on the surface of an ink-receiving layer, so that bleeding occurs, resulting in deterioration in print quality. 4) High-speed printing requires to have good drying ability. However, absorbing rate is insufficient, and the printed surface is hence not dry when discharged out of a printing apparatus, so that output images may be possibly impaired by contact. 5) There is a problem that the solids concentration of a dispersion of a pigment or the like cannot be increased because the viscosity of the dispersion increases with time, resulting in a failure to apply it. As a measure for the solution of the problem, Japanese Patent Application Laid-Open No. 4-67986 discloses a process in which the polymerization degree of a polymer as a binder is lowered. However, this process involves problems of defective appearance such as cracking in an ink-receiving layer, reduction in water fastness, and the like, and hence still requires a further improvement. 6) There is a problem that since the viscosity of the dispersion is high, its solids concentration cannot be increased. As a measure for the solution of the problem, Japanese Patent Application laid-Open No. 4-67985 discloses a process in which an acid such as a monocarboxylic acid is added as a dispersant. However, this process is accompanied by productive problems that offensive odor is given, and corrosion is caused. 7) In order to improve ink absorptiveness and resolution of images, U.S. Patent Nos. 4,780,356, 4,374,804 and 5,104,730, Japanese Patent Publication No. 3-72460, and Japanese Patent Application Laid-Open Nos. 55-11829, 58-110287, 62-270378 and 4-37576 disclose a process in which an ink-receiving layer of a two or more multi-layer structure is formed. However, the process involves a problem that coating and drying must be conducted at least twice for forming the ink-receiving layer, and so the number of processes increases. In addition, since the physical property values of the individual layers are different from each other, there are also problems of changes with time, defective appearance such as cracking in the ink-receiving layer, and separation and peeling of the layers from each other upon printing or the like. 8) Japanese Patent Application Laid-Open No. 3-281384 discloses an alumina hydrate forming an aggregate like a needle, which is in the form of a column having an aspect ratio of not higher than 3 and unidirectionally oriented, and a process for forming an ink-receiving layer using the alumina hydrate. However, since particles of the alumina hydrate are oriented and compacted, spaces among the alumina hydrate particles in the ink-receiving layer tends to narrow. Therefore, the pore radius is partial to a narrow side, and distribution of pore radius has a tendency to narrow. As a result, there is a problem that beading occurs as described above. SUMMARY OF THE INVENTION The present invention has thus been made with a view toward solving the above problems and has as its object the provision of a recording medium which can be suited to inks of various compositions, is excellent in ink absorptiveness, can suppress feathering or bleeding of print and occurrence of beading and can provide images high in optical density, and an ink-jet recording method using this recording medium. Another object of the present invention is to provide a recording medium which can effectively adsorb or absorb a dye and a solvent, which are components for an ink, and hence permits good coloring and quick drying, and an ink-jet recording method using this recording medium. A further object of the present invention is to provide a dispersion of a pigment, which is suitable for use in production of the above recording media. A still further object of the present invention is to provide a recording medium in which both dispersibility of a dispersion for coating and dye-adsorbing ability are satisfied, an ink-jet recording method using the recording medium, and a dispersion suitable for use in production of the recording medium. The above objects can be achieved by the present invention described below. According to the present invention, there is provided a recording medium comprising an alumina hydrate having at least two peaks in pore radius distribution, one of the peaks being located at smaller than 10 nm (100 Å) and the other being within a range of from 10 to 20 nm (100 to 200 Å), said alumina hydrate being defined by the following general formula: Al 2 O 3-n (OH) 2n · mH 2 O, wherein n is an integer of 0, 1, 2, or 3 and m is a number of 0 to 10, provided that m and n are not 0 at the same time. According to the present invention, there is also provided a recording medium comprising a base material and an ink-receiving layer which comprises a pigment and a binder and is provided on the base material wherein the pigment comprises an alumina hydrate having a chemical structure as defined above and the ink-receiving layer has at least two peaks in pore radius distribution one of the peaks being located at smaller than 10 nm (100 Å) and the other being within a range of from 10 to 20 nm (100 to 200 Å). In addition, according to the present invention, there is provided a recording medium comprising principally pulp fibers and a filler, wherein the filler comprises an alumina hydrate as recited above. Furthermore, according to the present invention, there is provided an ink-jet recording method comprising ejecting minute droplets of an ink from an orifice to apply the droplets to a recording medium, thereby conducting printing, wherein the recording medium as defined above is used as the recording medium. Moreover, according to the present invention, there is provided a dispersion of an alumina hydrate, which is obtained by dispersing an alumina hydrate as recited above containing 0.1 to 1.0 % by weight of a nitrate anion in deionized water, wherein the dispersion having a solids concentration of 15 % by weight has a viscosity of not higher than 75 mPa·s (cP) as measured at 20°C and a shear rate of 7.9 s -1 . The present invention also provides a process for the preparation of a recording medium comprising applying on a base material or adding internally into a fibrous material an alumina hydrate in the form of an aqueous dispersion, wherein the alumina hydrate is as recited above. Fig. 1 is a cross-sectional view of a recording medium according to the present invention. Fig. 2 diagrammatically illustrates an X-ray diffraction pattern of an alumina hydrate according to the present invention. Fig. 3 is a photograph illustrating a crystal structure of the alumina hydrate according to the present invention. Fig. 4 is a photograph illustrating the structure of particles of the alumina hydrate in an ink-receiving layer according to the present invention, viewed from the section of the ink-receiving layer. Fig. 5 diagrammatically illustrates an isothermal adsorption curve of an alumina hydrate used in the present invention. Fig. 6 diagrammatically illustrates the pore radius distribution of the alumina hydrate used in the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is directed to a recording medium comprising, as an essential component, at least an alumina hydrate having the above-described feature in pore radius distribution. The recording medium is constituted by, for example, internally containing its alumina hydrate in a sheet such as paper at a stage in which the sheet is formed from a raw material, or forming an ink-receiving layer composed principally of the alumina hydrate and a binder on a base material as illustrated in Fig. 1. The alumina hydrate is most preferable, in particular, as a material used in the ink-receiving layer because it has a positive charge, so that a dye in an ink is well fixed and an image good in coloring is hence provided, and moreover there are no problems of bronzing and light fastness, which have heretofore been caused by the use of silica compounds. A phenomenon called "bronzing" in which the hue of a black recorded area looks brownish has presented itself, and a new problem has hence been offered. The alumina hydrate used in the recording media according to the present invention may preferably be non-crystalline as analyzed by the X-ray diffraction method. The alumina hydrate is defined by the following general formula: Al 2 O 3-n (OH) 2n ·mH 2 O wherein n is an integer of 0, 1, 2 or 3, m is a number of 0 to 10, preferably 0 to 5. In many cases, mH 2 O represents an aqueous phase which does not participate in the formation of a crystal lattice, but is able to eliminate. Therefore, m may take a value other than an integer. Besides, m may take a value of 0 when a material of this kind is calcinated. The alumina hydrate can be produced by any conventional method such as the hydrolysis of aluminum alkoxide or sodium aluminate. Rocek, et al. [Collect Czech. Chem. Commun., Vol. 56, 1253-1262 (1991)] have reported that the pore structure of aluminum hydroxide is affected by deposition temperature, pH of the solution, aging time and a kind of surfactants used. For example, KODANSHA LTD PUBLISHERS, "Shokubai Koza (The Catalyst Course)", Vol. 5, Chapter on Engineering, Design of Catalysts, 123 (1985) describes an alumina hydrate as generally having one peak in pore radius distribution. Further, Kobayashi ["Surface", Vol. 15, 282 (1977)] has also reported that an alumina hydrate has one peak in pore radius distribution. Rocek, et al. [Collect Czech. Chem. Commun., 56(6), 1263-1269 (1991)] have reported that aluminum hydroxide deposited at a relatively low temperature of 30 to 50°C and pH 7 to 8 shows a two-peak porous structure having peaks in a large pore region and a medium pore region. The shape of the alumina hydrate used in the present invention is preferably in the form of a flat plate and has an average aspect ratio of 3 to 10 and a slenderness ratio of a flat plate surface of 0.6 to 1.0. The definition of the aspect ratio can be given by the method described in Japanese Patent Publication No. 5-16015. The aspect ratio is expressed by a ratio of "diameter" to "thickness" of a particle. The term "diameter" as used herein means a diameter of a circle having an area equal to a projected area of the particle, which has been obtained by observing the alumina hydrate through a microscope or a TEM. The slenderness ratio means a ratio of a minimum diameter to a maximum diameter of the flat plate surface when observed in the same manner as in the aspect ratio. If the average aspect ratio is lower than the lower limit of the above range, the range of the pore radius distribution of the ink-receiving layer narrows. On the other hand, average aspect ratios higher than the upper limit of the above range makes it difficult to produce the alumina hydrate with its particle size even. If the average slenderness ratio is lower than the lower limit of the above range, the range of the pore radius distribution similarly narrows. As described in the literature [Rocek J., et al., Applied Catalysis, Vol. 74, 29-36 (1991)], it is generally known that pseudoboehmite among alumina hydrates has both needle form and another form. According to a finding of the present inventor, the alumina hydrate in a flat plate form is more preferred because it has better dispersibility than that of a needle form (the ciliary form), and the orientation of particles of the alumina hydrate becomes random as illustrated in the photograph in Fig. 4 when forming an ink-receiving layer, so that the range of the pore radius distribution widens. In Handbook of Paper Processing, p. 402, there is described particles of aluminum hydroxide as being in the form of a hexagonal flat plate. Besides this, alumina hydroxides such as HIGILITE (registered trade name, product of Showa Denko K.K.) and HYDRAL (registered trade name, product of ALCOA), are known. Further, Japanese Patent Application Laid-Open No. 2-276670 discloses an alumina sol in the form of a needle having an aspect ratio of 2 to 10, while Japanese Patent Application Laid-Open No. 3-285814 discloses a boehmite sol in the form of a plate having an aspect ratio of 2 to 10. However, none of these documents disclose a relationship between the shape of the alumina hydrate and the pore structure or dispersibility of particles thereof as described below. The BET surface area of the alumina hydrate, the pore radius distributions of the alumina hydrate and the ink-receiving layer containing the alumina hydrate, and pore volumes and isothermal adsorption and desorption curves as described below can be obtained at the same time by the nitrogen adsorption and desorption method. The BET specific surface area of the alumina hydrate used in the present invention may preferably be within a range of from 70 to 300 m 2 /g. If the BET specific surface area is smaller than the lower limit of the above range, the pore radius distribution is partial to a large side. As a result, a dye in an ink cannot be fully adsorbed and fixed. On the other hand, surface areas greater than the upper limit of the above range result in failures to apply the pigment with good dispersibility and hence to control the pore radius distribution. No particular limitation is imposed on the production process of the alumina hydrate used in the present invention. However, it is preferable to use a process capable of producing a non-crystal alumina hydrate. For example, either of the Bayer's process or the alum pyrolysis process may be used. As a process for the production of the non-crystal alumina hydrate, which can be particularly preferably used in the present invention, may be mentioned a process in which an acid is added to an aluminum long-chain alkoxide to hydrolyze the alkoxide, thereby obtaining an alumina hydroxide. The term "aluminum long-chain alkoxide" as used herein means an alkoxide having, for example, 5 or more carbon atoms. Further, the use of an alkoxide having 12 to 22 carbon atoms is preferred because the removal of alcohol formed and the shape control of the alumina hydrate can be conducted with ease. The process has an advantage that impurities such as various ions are hard to get mixed compared with the process for producing alumina hydrogel or cationic alumina. The aluminum long-chain alkoxide also has an advantage that since the alcohol formed is easy to remove after the hydrolysis, the removal of the alcohol from the alumina hydrate can be completely conducted compared with the case where a short-chain alkoxide such as aluminum isopropoxide is used. Further, in the process making use of the aluminum long-chain alkoxide, particles of the alumina hydrate obtained by the hydrolysis tend to be in the form of a flat plate, and so the shape of the particles is easy to control. In this process, it is preferable from a viewpoint of obtaining the non-crystal alumina hydrate to preset the pH of a solution to 6 or lower upon the initiation of the hydrolysis. If the pH is more than 8, the alumina hydrate to be finally obtained will become crystalline. Dispersions of the above-described alumina hydrate used in the present invention may be used either as an additive (for example, added to a pulp slurry upon the preparation of a raw material for paper making) upon the production of the respective recording media, more specifically, upon the production of paper, or as a coating dispersion for forming an ink-receiving layer on a base material. In the recording medium used in the present invention according to an embodiment of the present invention, a coating dispersion (a dispersion of the alumina hydrate) containing the alumina hydrate as a pigment and a binder is applied to a base material to form an ink-receiving layer. The values of physical properties of the ink-receiving layer are not determined only by the alumina hydrate, but changed by various production conditions such as the kind and mixing amount of the binder, the concentration, viscosity and dispersion state of the coating dispersion, coating equipment, coating head, coating weight, and the flow rate, temperature and blowing direction of drying air. It is therefore necessary to control the production conditions within the optimum limits for achieving the properties of the ink-receiving layer according to the present invention. According to the present invention, the pore volume of the ink-receiving layer is preferably within a range of from 0.4 to 0.6 cm 3 /g (cc/g). If the pore volume of the ink-receiving layer is greater than the upper limit of the above range, cracking and dusting occur on the ink-receiving layer. If the pore volume is smaller than the lower limit of the above range, the resulting recording medium is deteriorated in ink absorption. The total pore volume of the ink-receiving layer is more preferably at least 8 cm 3 /m 2 (cc/m 2 ). If the total pore volume is smaller than this limit, inks tend to run out of the ink-receiving layer when multi-color printing is conducted, and so bleeding occurs on images. As with the ink-receiving layer, the pore volume of the alumina hydrate making up the ink-receiving layer is preferably within a range of from 0.4 to 0.6 cm 3 /g (cc/g). If the pore volume of the alumina hydrate is outside the above range, the pore volume of the ink-receiving layer cannot be controlled within the above range. The alumina hydrate obtained by the above process is subjected to a hydrothermal synthesis, thereby its particles grow (an aging step). By controlling the conditions of this step, the pore form of the alumina hydrate particles can be controlled within a specified range. Upon setting an aging time suitable, primary particles of the alumina hydrate having relatively even particle size grow, so that the sizes of spaces among the primary particles, which define pores, become even and the range of pore radius distribution becomes narrow. If the aging time is made longer than that conditions, the alumina hydrate having two tops in the pore radius distribution can be obtained. As a result, a recording medium, in which two tops are made in the pore radius distribution of alumina hydrate, can be obtained. The sol obtained may be used as a dispersion as it is as disclosed in Japanese Patent Application Laid-Open No. 2-276670. In the present invention, it is however preferable to dry the sol once into powder by a method such as spray drying and then prepare a dispersion. In this case, the dispersibility of the alumina hydrate in water is more enhanced. The alumina hydrate obtained by the process has two or more peaks in a pore radius distribution. A pore structure is mainly formed by spaces among the primary particles of the alumina hydrate. The reason why the pore radius distribution has at least two peaks is considered to be as follows. Since the alumina hydrate particles are in the form of a flat plate and the primary particles are oriented at random in the dried powder, there are defined spaces of portions in which primary particles overlap with each other in a direction of the principal plain of the flat plate and spaces of another portions in which an end face and the principal plane or another end face overlap with each other. Thus, two or more peaks in the pore radius distribution are all caused by the spaces among the primary particles, and at least one of the pore radii at one peak becomes smaller than the minor axis or major axis radius of the flat plate surface of the primary particles, while at least one of the pore radii at another peaks becomes about several times the minor axis or major axis radius of the flat plate surface. The alumina hydrate of the present invention, as described above, has at least two peaks in the pore radius distribution. Although such the alumina hydrate is dispersed up to a level of primary particles in a dispersion for coating, the radius distribution having at least one peaks is kept substantially even when the ink-receiving layer is formed through the steps of dispersing of the alumina hydrate, coating onto a substrate and drying. The reason is considered as follows: Even in an ink-receiving layer, the primary particles of the alumina hydrate are oriented in random direction, as shown by the photograph in Fig. 3, and therefore, as same as in a case of titanium dioxide-containing alumina hydrate, there are generated spaces of portions in which primary particles overlap with each other in a direction of the principal plane of the flat plate through a binder and spaces of another portions in which an end face and the principal plane or another end fade overlap with each other through a binder, by which the pore radius distribution having two or more peaks is kept even when the ink-receiving layer is formed therefrom. Japanese Patent Application Laid-Open No. 58-110287 discloses a recording sheet having peaks of pore radius at 0.05 µm or smaller and a range of from 0.2 to 10 µm. However, the former peak is caused by the spaces among primary particles, while the latter peak is defined by the spaces among secondary, tertiary or higher-order particles aggregated by the primary particles. Therefore, this sheet is different from the recording medium according to the present invention, in which said at least two peaks are caused by the spaces among primary particles. Accordingly, the positions of the peaks of the pore radii are quite different from those of the present invention. In the recording medium according to the present invention, the ink-receiving layer also has at least two peaks in the pore radius distribution. The solvent component in an ink is absorbed by relatively large pores, while the dye in the ink is adsorbed by relatively small pores. The pore radius corresponding to one of the peaks is preferably smaller than 10 nm (100 Å), more preferably 1 to 6 nm (10 to 60 Å). The pore radius corresponding to another peak is preferably within a range of from 10 to 20 nm (100 to 200 Å). If the pore radius corresponding to the former peak is larger than the above limit, the resulting recording medium is deteriorated in the adsorption and fixing of the dye in the ink, and so bleeding and beading occur on images. If the pore radius corresponding to the latter peak is smaller than the lower limit of the above range, the resulting recording medium is deteriorated in the absorption of the solvent component in the ink, so that the ink is not well dried, and the surface of the ink-receiving layer remains wet even when the medium is discharged out of a printer after printing. If the pore radius corresponding to the latter peak is greater than the upper limit of the above range, the resulting ink-receiving layer tends to crack. In the present invention, the alumina hydrate also has at least two peaks in the pore radius distribution. As with the ink-receiving layer, in the pore radius distribution, the pore radius corresponding to one of relatively small peaks is preferably smaller than 10 nm (100 Å), more preferably 1 to 6 nm (10 to 60 Å). The pore radius corresponding to a relatively large peak is preferably within a range of from 10 to 20 nm (100 to 200 Å). As described above, the pore structure is defined by the primary particles of the alumina hydrate. The nature of this pore structure is already created by the alumina hydrate. Therefore, if the pore radii corresponding to the peaks of the pore radius distribution of the alumina hydrate are outside the above ranges, the pore radii corresponding to the peaks of the pore radius distribution of the ink-receiving layer cannot be controlled within the above range. The pore volume of the ink-receiving layer is preferably within a range of from 0.1 to 1.0 cm 3 /g (cc/g), more preferably from 0.4 to 0.6 cm 3 /g (cc/g). If the pore volume of the ink-receiving layer is greater than the upper limit of the above range, cracking and dusting occur on the ink-receiving layer. If the pore volume is smaller than the lower limit of the above range, the resulting recording medium is deteriorated in ink absorption. The total pore volume of the ink-receiving layer is more preferably at least 8 cm 3 /m 2 (cc/m 2 ). If the total pore volume is smaller than this limit, inks tend to run out of the ink-receiving layer when multi-color printing is conducted, and so bleeding occurs on images. The pore volume of pores having a peak at a pore radius of smaller than 10 nm (100 Å) is preferably within a range of from 0.1 to 10 % by volume, more preferably from 1 to 5 % by volume based on the total pore volume. The pore volume of the pores having the peak at a pore radius of smaller than 10 nm (100 Å) means a pore volume within a range showing a breadth of pore radii having a magnitude half of the greatest-magnitude pore radius of the pores having a peak at smaller than 10 nm (100 Å) in the pore radius distribution. If the pore volume of the pores having a peak at a pore radius of smaller than 10 nm (100 Å) in the ink-receiving layer is smaller than the lower limit of the above range, the resulting recording medium is deteriorated in adsorption of a dye in an ink. If the pore volume exceeds the upper limit of the above range, the resulting recording medium is deteriorated in the absorption of the solvent component in the ink. The pore volume of the alumina hydrate is also preferably within a range of from 0.1 to 1.0 cm 3 /g (cc/g), more preferably from 0.4 to 0.6 cm 3 /g (cc/g). The pore volume of pores having a peak at a pore radius of smaller than 10 nm (100 Å) is preferably within a range of from 0.1 to 10 % by volume, more preferably from 1 to 5 % by volume based on the total pore volume. The pore volume of the ink-receiving layer depends upon the pore volume of the alumina hydrate. Therefore, if the pore volume of the alumina hydrate is outside the above range, the pore volume of the ink-receiving layer cannot be controlled within the above range. In the recording media according to the present invention, which each have an ink-receiving layer on a base material, a relative pressure difference (ΔP) between adsorption and desorption at 90 percent of the maximum amount of adsorbed gas as found from an isothermal nitrogen adsorption and desorption curve for the ink-receiving layer, which is derived from the nitrogen adsorption and desorption method, is preferably not larger than 0.2, more preferably not larger than 0.15, most preferably not larger than 0.10. As described in McBain [J. Am. Chem. Soc., Vol. 57, 699 (1935)], the relative pressure difference (ΔP) can be used as an index whether a pore in the form of an inkpot may exist. The pore is closer to a straight tube as the relative pressure difference (ΔP) is smaller. On the other hand, the pore is closer to an inkpot as the difference is greater. Differences exceeding the above limit result in a recording medium poor in dryness of an ink after printing. Japanese Patent Application Laid-Open No. 60-245588 describes the fact that with respect to the shape of pores in an alumina xerogel used in an ink-receiving layer, those low in degree of labyrinth, even and linear are preferred, while an inkpot form narrow in inlet, a gourd form constricted in the middle and a winding form are not preferred from the viewpoint of rate of absorption. However, this publication does not disclose anything about specific methods for measuring actual physical properties and the like. With respect to the above-described alumina hydrates in the recording media according to the present invention, a relative pressure difference (ΔP) between adsorption and desorption at 90 percent of the maximum amount of adsorbed gas as found from an isothermal nitrogen adsorption and desorption curve for each of the alumina hydrates, which is derived from the nitrogen adsorption and desorption method, is preferably not larger than 0.2, more preferably not larger than 0.15, most preferably not larger than 0.10. If the difference is outside this limit, the relative pressure difference (ΔP) of the ink-receiving layer as found from the isothermal nitrogen adsorption and desorption curve cannot be controlled within the above limit. In the recording media according to the present invention, the number of hydroxyl groups on the surface of each of the alumina hydrates is preferably at least 10 20 groups/g. If the number is fewer than this value, the solids concentration of a dispersion in which the alumina hydrate is dispersed in water cannot be increased. Such number of hydroxyl groups on the surface of the alumina hydrate can be determined by the titration with a triethylaluminum solution. The surface potential of each of the alumina hydrates used in the present invention can be determined by a zeta potential analyzer. Japanese Patent Application Laid-open No. 60-232990 discloses that alumina compounds have a positive charge, and in its Examples, the values of zeta-potentials. However, specific measuring method and conditions are not described therein. The value of a zeta-potential varies depending upon the cell and electrode structure in a measuring apparatus, applied voltage, the solids concentration and pH of a dispersion, and dispersants and additives used. Therefore, absolute values cannot be directly compared unless the measuring conditions, apparatus and the like are standardized to conduct the measurement. With respect to the alumina hydrates used in the present invention, the zeta-potential is preferably at least 15 mV as measured at pH 6 in the form of a 0.1 % by weight aqueous dispersion free from any dispersant and additive. If the zeta-potential is above this limit, the alumina hydrate can be easily dispersed up to a level of primary particles in a dispersion. If the zeta-potential is lower than 15 mV, aggregate and deposit occur as the solids concentration increases, or particles partly aggregate to form great lumps when a binder dispersion is mixed with the alumina hydrate. For this reason, in particular, in the recording media having an ink-receiving layer, pore radius of the ink-receiving layer becomes markedly large, and so the strength of the ink-receiving layer is lowered, resulting in a potential problem that dusting may occur, or dye-fixing ability upon printing may be deteriorated. In general, alumina hydrate is stable in a low pH region. It is therefore known to add an acid to low the pH of the dispersion. However, the addition of the acid is not preferred from the viewpoint of the occurrence of offensive odor and corrosion and of the limitation of the kind of binder used. Further, a known process in which a dispersant is added is not preferred because repellent or the like occurs upon the coating of the dispersion. On the other hand, if the pH region becomes higher, primary particles aggregate according to the kind of the alumina hydrate to enlarge the particle size, so that the alumina hydrate may apparently have a high zeta-potential in some cases. The zeta-potential recited in the present invention should be measured in a condition that such aggregation of particles does not occur. In order to determine whether such aggregation of particles occurs or not, it is effective to measure the particle size of dispersed particles. Any known method may be used as a measuring method of the particle size. It is necessary to confirm that the particle size of particles in a dispersion kept at pH 6, which is subjected to the measurement of zeta-potential, has substantially the same value as the particle size of particles in a dispersion kept at pH 4 in which the particles are said to be stably dispersed. In the present invention, a dispersion obtained by dispersing an alumina hydrate containing 0.1 to 1.0 % by weight of a nitrate anion, said alumina hydrate being such a specific alumina hydrate as described above, in deionized water to give a solids concentration of 15 % by weight preferably has a viscosity of not higher than 75 mPa·s (cP), most preferably not higher than 30 mPa·s (cP) as measured at 20°C and a shear rate of 7.9 s -1 (sec -1 ). Further, a dispersion obtained by dispersing the same alumina hydrate containing 0.1 to 1.0 % by weight of a nitrate anion as described above in deionized water to give a solids concentration of 20 % by weight preferably has a viscosity of not higher than 100 mPa·s (cP), most preferably not higher than 80 mPa·s (cP) as measured at 20°C and a shear rate of 10.2 s -1 (sec -1 ). Furthermore, a dispersion obtained by dispersing the same alumina hydrate containing 0.1 to 1.0 % by weight of a nitrate anion as described above in deionized water to give a solids concentration of 25 % by weight preferably has a viscosity of not higher than 500 mPa·s (cP), most preferably not higher than 460 mPa·s (cP) as measured at 20°C and a shear rate of 10.2 s -1 (sec -1 ). In each of the above cases, if the viscosity exceeds the upper limit, the dispersion is required to low its solids concentration. It is not hence preferable from the viewpoint of mass productivity to increase the viscosity beyond the above limit. The viscosities of the alumina hydrate dispersions according to the present invention can be measured by means of a rotational viscometer, for example, a Brookfield type viscometer. The prior art making use of the pseudoboehmite cited above and the present invention have been investigated in comparison with each other. As a result, differences therebetween are as follows: 1) With respect to the pore radius distribution in the prior art, the breadth of the pore radius distribution is disclosed only in a narrow range as demonstrated by the fact that the pore volume of pores within a range of the average pore radius ± 1 nm (10 Å) amounts to 45 to 55 % of the total pore volume. On the contrary, the present inventor has found that when the breadth of the pore radius distribution is widened, print quality remains unchanged even if the proportion of a dye to a solvent in an ink and the composition of materials are changed. It has also been found that when one gets the pore radius distribution to have at least two peaks to divide the function of the pores, the solvent component in an ink is absorbed in pores having a relatively large radius, while the dye component in the ink is adsorbed on pores having a relatively small radius, whereby a recording medium, by which inks are quickly dried and good coloring is achieved even when a multi-color printing is conducted at high speed, is provided. 2) With respect to the pore radius and pore volume, the prior art describes the total pore volumes as being 0.2 to 1.0 cm 3 /g (cc/g) for pores having a pore radius of 1 to 4 nm (10 to 40 Å), 0.1 to 0.4 cm 3 /g (cc/g) for pores having a pore radius of 4 to 10 nm (40 to 100 Å), and 0.1 cm 3 /g (cc/g) for pores having a pore radius of 10 to 100 nm (100 to 1000 Å). On the contrary, the present inventor has found that when the relationship between the pore volume per unit area of the ink-receiving layer and the isothermal nitrogen adsorption and desorption curve thereof is adjusted within the range described herein, the adsorption and drying of the ink after printing are markedly improved. 3) The prior art describes a pseudoboehmite sol in the form of a needle and a production process of an ink-receiving layer making use of the sol. The shape of the alumina hydrate and the solids concentration of a dispersion are also described. On the contrary, the present inventor has found the use of non-crystal alumina hydrates in the form of "a flat plate". As described above, such a non-crystal alumina hydrate in the form of a flat plate is produced by hydrolyzing an aluminum long-chain alkoxide. Therefore, it is possible to obtain an alumina hydrate containing little ion and raw alcohol with ease. According to this hydrolysis, the shape control for forming particles of the alumina hydrate into the flat plate can be made with ease. The non-crystal alumina hydrate in the flat plate form has higher dispersibility than the known alumina hydrate in the needle form. Further, the resulting alumina hydrate is dried once to powder without directly preparing a dispersion in a sol state, and then used, whereby a dispersion high in solids concentration and low in viscosity can be prepared with ease. In the recording medium according to the present invention, a binder capable of using in combination with the alumina hydrate may be freely selected from water-soluble polymers. For example, preference may be given to polyvinyl alcohol or modified products thereof (cationically modified, anionically modified, silanol modified), starch or modified products thereof (oxidized, etherified), casein or modified products thereof, gum arabic, cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose, conjugated diene copolymer latices such as SBR latices, NBR latices and methyl methacrylate-butadiene copolymers, functional group-modified polymer latices, vinyl copolymer latices such as ethylene-vinyl acetate copolymers, polyvinyl pyrrolidone, maleic anhydride polymer or copolymers thereof, acrylic ester copolymers, and the like. These binders may be used either singly or in any combination thereof. The mixing ratio of the alumina hydrate to the binder may be optionally selected from a range of from 1:1 to 30:1, preferably from 5:1 to 25:1. If the amount of the binder is less than the lower limit of the above range, the mechanical strength of the resulting ink-receiving layer is insufficient, which forms the cause of cracking and dusting. If the amount is greater than the upper limit of the above range, the pore volume of the resulting ink-receiving layer is reduced, resulting in a recording medium poor in ink absorptiveness. The alumina hydrate and binder may optionally contain dispersants for the alumina hydrate, viscosity modifiers, pH adjustors, lubricants, flowability modifiers, surfactants, antifoaming agents, water-proofings, foam suppressors, releasing agents, foaming agents, penetrants, coloring dyes, optical whitening agents, ultraviolet absorbents, antioxidants, antiseptics and mildewproofing agents. The water-proofings may be freely selected for use from the known substances such as quaternary ammonium halides and quaternary ammonium salt polymers. As the base material constituting the recording medium of the present invention, may be used paper webs such as suitably sized paper, water leaf paper and resin-coated paper making use of polyethylene or the like, sheet-like substance such as thermoplastic films, and cloths. No particular limitation is imposed on the base material. In the case of the thermoplastic films, may be used transparent films such as films of polyester, polystyrene, polyvinyl chloride, polymethyl methacrylate, cellulose acetate, polyethylene and polycarbonate, as well as opaque sheets opacified by the filling of an alumina hydrate or the formation of minute foams. A dispersion of the alumina hydrate, which is suitably used in the production of the recording medium according to the present invention, can be prepared in the following manner. The above-described powdery alumina hydrate is added to deionized water to prepare a dispersion containing solids at a desired concentration. Mechanical shear force or ultrasonic energy is applied to the dispersion, as needed, to control the particle size of the alumina hydrate. Thereafter, a binder dispersion separately prepared is added to the former dispersion. The resulting mixture is subjected to dispersion, heating, defoaming and/or the like as needed, thereby obtaining a final dispersion for coating. In the recording media having an ink-receiving layer according to the present invention, as a process for forming the ink-receiving layer on the base material, may be used a process in which the dispersion containing the alumina hydrate and the like is applied to the base material by means of a coater, and then dried. As a coating process, may be used a generally-used coating technique making use of a blade coater, air knife coater, roll coater, brush coater, curtain coater, bar coater, gravure coater or sprayer. The coating weight of the dispersion is within a range of from 0.5 to 60 g/m 2 , more preferably from 5 to 45 g/m 2 in dried state. As needed, the resulting recording medium may by subjected to supercalendering or the like so as to improve the smoothness of the ink-receiving layer. The recording media of another type according to the present invention, in which the alumina hydrate is internally contained in the base material, can be produced using an internally-adding process in which the alumina hydrate (its dispersion) is added to a slurry containing a fibrous material in a paper making process. In such an internally-adding process, a paper-strength improving agent, retention aid and colorant may be added for use as needed. The retention aid may be selected from cationic retention aids such as cationic starch and dicyanediamide-formalin condensates, and anionic retention aids such as anionic polyacrylamide and anionic colloidal silica. These may also be used in combination with each other. Inks used in conducting recording on the recording media according to the present invention comprises principally a coloring material (dye or pigment), a water-soluble organic solvent and water. Preferred examples of the coloring material include water-soluble dyes represented by direct dyes, acid dyes, basic dyes, reactive dyes and food colors. However, any coloring materials may be used so far as they provide images satisfying required performance such as fixing ability, coloring ability, brightness, stability, light fastness and the like in combination with the above-described recording media. The water-soluble dyes are generally used by dissolving them in water or a solvent composed of water and at least one organic solvent. As a preferable solvent component for these dyes, may be used a mixed solvent composed of water and at least one of various water-soluble organic solvents. It is however preferable to control the content of water in an ink within a range of from 20 to 90 % by weight, more preferably from 60 to 90 % by weight. Examples of the water-soluble organic solvents include alkyl alcohols having 1 to 4 carbon atoms, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol and isobutyl alcohol; amides such as dimethylformamide and dimethylacetamide; ketones and keto alcohols such as acetone and diacetone alcohol; ethers such as tetrahydrofuran and dioxane; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; alkylene glycols the alkylene moiety of which has 2 to 6 carbon atoms, such as ethylene glycol, propylene glycol, hexylene glycol and diethylene glycol; thiodiglycol; 1,2,6-hexanetriol; glycerol; lower alkyl ethers of polyhydric alcohols, such as ethylene glycol methyl ether, diethylene glycol methyl ether, diethylene glycol ethyl ether, triethylene glycol monomethyl ether and triethylene glycol monoethyl ether; and the like. Among these many water-soluble organic solvents, the polyhydric alcohols such as diethylene glycol, and the lower alkyl ethers of polyhydric alcohol, such as triethylene glycol monomethyl ether and triethylene glycol monoethyl ether are preferred. The polyhydric alcohols are particularly preferred because they have an effect as a lubricant for inhibiting the clogging of nozzles, which is caused by the evaporation of water in an ink and hence the deposition of a water-soluble dye. A solubilizer may be added to the inks. Nitrogen-containing heterocyclic ketones are typical solubilizers. Its object is to enhance the solubility of the water-soluble dye in the solvent by leaps and bounds. For example, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone are preferably used. In order to further improve the properties of inks, may be added additives such as viscosity modifiers, surfactants, surface tension modifiers, pH adjustors and specific resistance adjustors. A preferred method of conducting recording by applying the above-described ink to the recording medium is an ink-jet recording method. As such a method, any systems may be used so far as they can effectively eject an ink out of a nozzle to apply it to the recording medium. In particular, an ink-jet recording system described in Japanese Patent Application Laid-Open No. 54-59936, in which an ink undergoes a rapid volumetric change by an action of thermal energy applied to the ink, so that the ink is ejected out of a nozzle by the working force generated by this change of state, may be used effectively. [Examples] The present invention will hereinafter be described more specifically by the following Examples. However, the present invention is not limited to these examples. The measurements of various properties according to this invention were conducted with the following points in mind: 1) BET specific surface area, pore radius distribution, pore volume and isothermal adsorption and desorption curve: After an alumina hydrate sample or a recording medium sample in which an ink-receiving layer had been formed on a PET film was thoroughly heated and deaerated, measurement was conducted using the nitrogen adsorption and desorption method (Autosorb 1, manufactured by Quanthachrome Co.). i) The BET specific surface area was calculated in accordance with the method of Brunauer, et al. [J. Am. Chem. Soc., Vol. 60, 309 (1938)]. ii) The pore radius and pore volume were calculated in accordance with the method of Barrett, et al. [J. Am. Chem. Soc., Vol. 73, 373 (1951)]. iii) The relative pressure difference (ΔP) between adsorption and desorption at 90 percent of the maximum amount of adsorbed gas was found from an isothermal nitrogen adsorption and desorption curve. 2) X-ray diffraction pattern: An X-ray diffractometer (manufactured by RIGAKU CORPORATION) was used. 3) Shape observation of alumina hydrate (aspect ratio and slenderness ratio): An alumina hydrate sample was dispersed in deionized water, and the resultant dispersion was dropped on a collodion membrane to prepare a sample for measurement. This sample was observed through a transmission type electron microscope (H-500 (registered trademark), manufactured by Hitachi Ltd.). 4) Number of hydroxyl groups: One gram of an alumina hydrate sample was weighed out to titrate it with triethylaluminum. 5) Zeta-potential: An alumina hydrate sample was dispersed in deionized water to give a solids concentration of 0.1 % by weight, and the dispersion was then adjusted to pH 6 with a nitric acid, thereby conducting measurement. (Bi-ZETA plus (registered trademark), manufactured by Brookheaven Co., dispersion temperature: 20°C, an acrylic cell was used.) 6) Solution viscosity: An aqueous dispersion sample at solids concentration of 15 % by weight, 20 % by weight or 25 % by weight was prepared to measure its viscosity at 20°C and a shear rate of 7.9 s -1 (sec -1 ) (in the case of the solids concentration of 15 % by weight) or 10.9 s -1 (sec -1 ) (in the case of the solids concentration of 20 % by weight or 25 % by weight) by means of a VISCOMETER (registered trademark) manufactured by TOKIMEC Co. 7) Nitrate anion: A nitrate anion was extracted from an alumina hydrate sample with hot water to measure its quantity by an ion-exchange chromatograph (L-3720 (registered trademark), manufactured by Hitachi Ltd.), thereby determining the quantity of the nitrate anion in terms of % by weight of dried alumina hydrate. 8) Quantitative analysis of titanium dioxide: The content of titanium dioxide in the whole alumina hydrate sample was determined by fusing the alumina hydrate sample in a borate in accordance with the ICP method (SPS 4000, manufactured by Seiko-Electronic Inc.). The distribution of titanium dioxide in the alumina hydrate sample was analyzed by means of an ESCA (Model 2803, manufactured by Surface Science Instruments Co.). The surface of the alumina hydrate sample was etched with an argon ion for 100 seconds and 500 seconds to determine the change in content of the titanium dioxide. Etching conditions was as follows: Argon pressure: 5 x 10 -4 Pa Voltage applied: 3 kV Current (d.c.) applied: 3 mA. Examples 1 to 4 : Aluminum dodeoxide was prepared in accordance with the process described in U.S. Patent No. 4,242,271. The aluminum dodeoxide was then hydrolyzed in accordance with the process described in U.S. Patent No. 4,202,870 to prepare an alumina slurry. Water was added to the alumina slurry until the solid content of alumina hydrate was 7.9 %. The pH of the alumina slurry was 9.5. A 3.9 % nitric acid solution was added to adjust the pH of the slurry. Colloidal sols were obtained under their corresponding aging conditions shown in Table 1. Each of these colloidal sols was spray-dried at 75°C to obtain its corresponding alumina hydrate. These alumina hydrates showed an X-ray diffraction pattern similar to that illustrated in Fig. 2 and were hence amorphous. A photograph similar to the photograph (an electron microphotograph: 60,000 magnifications) in Fig. 3 was taken, and it was hence confirmed that they were in the form of a flat plate. The physical property values of the alumina hydrates were measured in accordance with the respective methods described above. The results are shown in Table 2 and Figs. 5 and 6. Aging condition Sample Ex. 13 Ex. 14 Ex. 15 Ex. 16 pH before aging 6.6 6.9 7.0 6.8 Aging temperature (°C) 30 45 50 50 Aging period 2 weeks 12 days 8 days 5 days Aging apparatus Oven Oven Oven Autoclave Polyvinyl alcohol (Gohsenol NH18 (registered trademark), product of The Nippon Synthetic Chemical Industry Co., Ltd.) was dissolved or dispersed in deionized water to obtain a 10 % by weight solution or dispersion. The four different kinds of alumina hydrates obtained above were separately similarly dispersed in deionized water to obtain 15 % by weight dispersions. Each of the alumina hydrate dispersions and the polyvinyl alcohol solution were weighed out to give a weight ratio of 10:1 in terms of solids and mixed with each other. The resultant mixture was stirred to obtain a mixed dispersion. The mixed dispersion was applied by a die coating process to one side of a PET film (Lumiror (registered trademark), product of Toray Industries, Inc.) having a thickness of 100 µm to form an ink-receiving layers having a thickness of 30 µm. Fig. 4 is a photograph (an electron microphotograph: 50,000 magnifications) illustrating the section of the ink-receiving layer and indicates that the alumina hydrate in the form of a flat plate lies at random in the ink-receiving layer. The physical property values of the ink-receiving layer were measured in accordance with the respective methods described above. The results are shown in Table 3. Printing was conducted on the thus-obtained recording media to evaluate their printability in accordance with the following standards. The evaluation results are shown in Table 4. Printability : Using an ink-jet printer equipped with four recording heads for yellow, magenta, cyan and black inks, each of said heads having 128 nozzles in a proportion of 16 nozzles per mm, ink-jet recording was conducted with inks of the following ink composition, thereby evaluating the recording media in ink-drying ability (absorptiveness), optical density of an image, bleeding and beading. (1) Ink-drying ability: After single-color or multi-color solid printing was conducted with the yellow, magenta, cyan and black inks of the following ink composition, the recorded area of each recording medium was touched with a finger to determine the drying condition of the inks on the surface of the recording medium. The quantity of ink in the single-color printing was determined as 100 %. The ink-drying ability was ranked as AA where none of the inks adhered to the finger in an ink quantity of 300%, A where none of the inks adhered to the finger in an ink quantity of 200 %, B where none of the inks adhered to the finger in an ink quantity of 100 %, or C where any ink adhered to the finger in an ink quantity of 100 %. (2) Optical density: Solid printing was conducted separately with the black ink of the following ink composition. The optical density of each of the images formed was determined by means of a Macbeth reflection densitometer RD-918 (registered trademark). (3) Bleeding and beading: After single-color or multi-color solid printing was conducted with the yellow, magenta, cyan and black inks of the following ink composition, the recording media were visually evaluated by whether bleeding occurred on their surfaces. The quantity of ink in the single-color printing was determined as 100 %. The resistance to bleeding or the resistance to beading of the recording media was ranked as AA where bleeding or beading did not occur in an ink quantity of 300%, A where bleeding or beading did not occur in an ink quantity of 200 %, B where bleeding or beading did not occur in an ink quantity of 100 %, or C where bleeding or beading occurred in an ink quantity of 100 %. Ink composition: Dye 5 parts Diethylene glycol 10 parts Polyethylene glycol 10 parts Water 75 parts. Dye in ink: Yellow (Y): C.I. Direct Yellow 86 Magenta (M): C.I. Acid Red 35 Cyan (C): C.I. Direct Blue 199 Black (Bk): C.I. Food Black 2. Item determined Sample Ex. 1 Ex. 2 Ex. 3 Ex. 4 Average particle size [nm] [(Å)] 4,0 (40) 3,5 (35) 3,1 (31) 2,8 (28) Aspect ratio 3.8 5.8 7.6 9.8 Slenderness ratio 0.7 0.7 0.7 0.7 BET specific surface area (m 2 /g) 80 95 130 190 Peak 1 of pore distribution [nm(Å)] 12,5 (125) 11,0 (110) 14,0 (140) 12,0 (120) Peak 2 of pore distribution [nm(Å)] 1,7 (17) 3,0 (30) 5,0 (50) 6,0 (60) Pore volume [cm 3 /g 0.57 0.55 0.55 0.51 (cc/g)] Volume ratio of peak 2 (%) 5 8 10 10 Relative pressure difference (ΔP) 0.03 0.02 0.10 0.10 Number of OH groups (groups/g) 1.9x10 20 1.5x10 20 1.7x10 20 2.0x10 20 Zeta-potential (mV) 20 23 17 17 Nitrate anion (% by weight) 0.1 0.5 0.7 1.0 Dispersion viscosity [mPa·s (cP)] 15 % by weight 18 18 17 16 20 % by weight 43 50 80 75 25 % by weight 415 430 480 467 Item determined Sample Ex. 1 Ex. 2 Ex. 3 Ex. 4 Peak 1 of pore distribution [nm(Å)] 12,5 (125) 11,0 (110) 12,5 (125) 13,0 (130) Peak 2 of pore distribution [nm(Å)] 2,5 (25) 3,0 (30) 4,0 (40) 6,0 (60) Pore volume (cm 3 /g) 0.53 0.50 0.50 0.49 (cm 3 /m 2 ) 9.0 8.6 8.4 8.2 Relative pressure difference (ΔP) 0.03 0.07 0.02 0.10 Volume ratio of peak 2 (%) 5 8 10 10 Printability: Drying ability AA AA AA AA Optical density 1.65 1.67 1.66 1.67 Bleeding AA AA AA AA Beading AA AA AA AA Examples 5 to 8 : Each of the alumina hydrate dispersions prepared in Examples 1 to 4 and the polyvinyl alcohol dispersion as described in Examples 1 to 4 were weighed out to give a weight ratio of 15:1 in terms of solids and mixed with each other. The resultant mixture was stirred to obtain a mixed dispersion. The mixed dispersion was applied by an air knife coating process to one side of a wood free paper web (Shiraoi 157 (registered trademark), product of Daishowa Paper Manufacturing Co., Ltd.) at a rate of 20 g/m 2 to form an ink-receiving layer. Printing was conducted on the thus-obtained recording media to evaluate their printability in accordance with the above-described standards. The evaluation results are shown in Table 5. Item determined Sample Ex. 5 Ex. 6 Ex. 7 Ex. 8 Printability: Drying ability AA AA AA AA Optical density 1.51 1.52 1.51 1.52 Bleeding AA AA AA AA Beading AA AA AA AA Examples 9 to 12 : A paper web having a basis weight of 70 g/m 2 was made by means of a TAPPI standard sheet former by using, as raw pulp, 80 parts of bleached hardwood kraft pulp (LBKP) having a freeness (C.S.F.) of 370 ml and 20 parts of bleached softwood kraft pulp (NBKP) having a freeness of 410 ml, incorporating, as a filler, each of the alumina hydrates prepared in Examples 1 to 4 in a proportion of 35 % by weight based on the solid content of the pulp and as a retention aid, cationic starch (CATOF (registered trademark), product of Oji National K.K.) in a proportion of 0.3 % by weight based on the solid content of the pulp into the pulp, and then adding 0.05 % by weight of a polyacrylamide retention aid (Pearlflock FR-X (registered trademark), product of Seiko Chemical Industries Co., Ltd.) right before paper making. A 2 % solution of oxidized starch (MS3800 (registered trademark), product of Nihon Shokuhin Kako Co., Ltd.) was then applied to the web by a size press to obtain a recording media. Printing was conducted on the thus-obtained recording media to evaluate their printability in accordance with the above-described standards. The evaluation results are shown in Table 6. Item determined Sample Ex. 9 Ex. 10 Ex. 11 Ex. 12 Printability: Drying ability AA AA AA AA Optical density 1.08 1.09 1.08 1.09 Bleeding AA AA AA AA Beading AA AA AA AA The recording medium according to the present invention and the ink-jet recording method making use of these recording media have the following advantageous effects. 1) Since the individual pigments or ink-receiving layers have at least two peaks in pore radius distribution, the function of the pores can be divided. 2) Since a dye in an ink is effectively adsorbed to pores having a relatively small radius, images good in resolution and sufficient in optical density can be provided. 3) Since a solvent component in the ink can be quickly absorbed in pores having a relatively large radius, images free of beading, bleeding and running of the ink and good in resolution can be provided. 4) Since the recording media have no hysteresis, the solvent component in the ink is easy to be desorbed. Therefore, the ink-drying ability of the media is improved, and so bleeding and setoff can be prevented. 5) When alumina in the form of a flat plate is used, the spaces among its particles can be widened if the closest packing is adopted. Therefore, there can be obtained a medium having pores considerably wide in pore radius distribution. 6) Since the alumina hydrate has good dispersibility, the viscosity of a dispersion can be kept low if the solids concentration of the dispersion is high. 7) Since the alumina hydrate has good dispersibility even at a neutral region near pH 7, the amount of an acid added to the dispersion can be decreased. The solvent component in the ink is easy to be desorbed. Therefore, the ink-drying ability of the media is improved, and so bleeding and setoff can be prevented. While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments.

Claims: A recording medium comprising an alumina hydrate having at least two peaks in pore radius distribution, one of the peaks being located at smaller than 10 nm (100 Å) and the other being within a range of from 10 to 20 nm (100 to 200 Å), said alumina hydrate being defined by the following general formula: Al 2 O 3-n (OH) 2n · mH 2 O, wherein n is an integer of 0, 1, 2, or 3 and m is a number of 0 to 10, provided that m and n are not 0 at the same time. A recording medium comprising a base material and an ink-receiving layer which comprises a pigment and a binder and is provided on the base material, wherein the pigment comprises an alumina hydrate having a chemical structure as defined in claim 1 and the ink-receiving layer has at least two peaks in pore radius distribution, one of the peaks is located at smaller than 10 nm (100 Å) and the other is within a range of from 10 to 20 nm (100 to 200 Å). A recording medium comprising principally pulp fibers and a filler, wherein the filler comprises an alumina hydrate as recited in claim 1. The recording medium according to claim 1 or 3, wherein the pore volume of pores having the peak at a pore radius of smaller than 10 nm (100 Å) in the alumina hydrate is within a range of from 0.1 to 10 % by volume based on the total pore volume. The recording medium according to claim 2, wherein the pore volume of pores having the peak at a pore radius of smaller than 10 nm (100 Å) in the ink-receiving layer is within a range of from 0.1 to 10 % by volume based on the total pore volume. The recording medium according to any of claims 1 to 3, wherein the pore volume of the alumina hydrate is within a range of from 0.4 to 0.6 cm 3 /g. The recording medium according to claim 2, wherein the pore volume of the ink-receiving layer is within a range of from 0.4 to 0.6 cm 3 /g. The recording medium according to claim 2 or 7, wherein the total volume of the ink-receiving layer is at least 8 cm 3 /m 2 . The recording medium according to any of claims 1 to 3, wherein a relative pressure difference (ΔP) between adsorption and desorption at 90 percent of the maximum amount of adsorbed gas as found from an isothermal nitrogen adsorption and desoprtion curve for the alumina hydrate is not larger than 0.2. The recording medium according to claim 2, wherein a relative pressure difference (ΔP) between adsorption and desorption at 90 percent of the maximum amount of adsorbed gas as found from an isothermal nitrogen adsorption and desorption curve for the ink-receiving layer is not larger than 0.2. The recording medium according to any one of claims 1 to 10, wherein the alumina hydrate is non-crystalline. The recording medium according to any of claims 1 to 11, wherein the alumina hydrate is in the form of a flat plate having an average aspect ratio of 3 to 10. The recording medium according to any of claims 1 to 12, wherein the alumina hydrate has a BET specific surface area of 70 to 300 m 2 /g. The recording medium according to any of claims 1 to 13, wherein the alumina hydrate has an average slenderness ratio of 0.6 to 1.0. The recording medium according to any of claims 1 to 14, wherein the number of hydroxyl groups in the alumina hydrate is at least 10 20 groups/g. The recording medium according to any of claims 1 to 15, wherein the alumina hydrate has a zeta-potential of at least 15 mV at pH 6. The recording medium according to claim 1 or 3, wherein a pore radius of one of the peaks is in a range of from 1 to 6 nm (10 to 60 Å). The recording medium according to claim 2, wherein a pore radius of one of the peaks is in a range of from 1 to 6 nm (10 to 60 Å). The recording medium according to claim 2, wherein the binder comprises a water-soluble polymer. The recording medium according to claim 19, wherein the water-soluble polymer is a material selected from the group consisting of polyvinyl alcohol, cationically-modified polyvinyl alcohol, anionically-modified polyvinyl alcohol, silanol-modified polyvinyl alcohol, starch, oxidized starch, etherified starch, casein, gum arabic, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, styrene-butadiene rubber, methylmethacrylate-butadien copolymer, ethylene-vinyl acetate copolymer, polyvinyl pyrrolidone, maleic anhydride polymer or copolymers thereof, and acrylic ester copolymer. The recording medium according to claim 2, wherein a mixing weight ratio of the alumina hydrate to the binder is in the range of from 1/1 to 30/1. The recording medium according to claim 21, wherein a mixing weight ratio of the alumina hydrate to the binder is in the range of from 5/1 to 25/1. An ink-jet recording method comprising ejecting minute droplets of an ink from an orifice to apply the droplets to a recording medium, thereby conducting printing, wherein the recording medium according to any of the preceding claims is used as the recording medium. The ink-jet recording method according to claim 23, wherein the minute droplets of the ink are formed by applying thermal energy to the ink. A dispersion of an alumina hydrate, which is obtained by dispersing an alumina hydrate as recited in claim 1 containing 0.1 to 1.0 % by weight of a nitrate anion in deionized water, wherein the dispersion having a solids concentration of 15 % by weight has a viscosity of not higher than 75 mPa·s (cP) as measured at 20°C and a shear rate of 7.9 s -1 . The alumina hydrate dispersion according to claim 25, wherein the dispersion has a viscosity of not higher than 100 mPa·s (cP) as measured at 20°C and a shear rate of 10.2 s -1 when the alumina hydrate is dispersed at a solids concentration of 20 % by weight. The alumina hydrate dispersion according to claim 26, wherein the dispersion has a viscosity of not higher than 500 mPa·s (cP) as measured at 20°C and a shear rate of 10.2 s -1 when the alumina hydrate is dispersed at a solids concentration of 25 % by weight. A process for the preparation of a recording medium comprising applying on a base material or adding internally into a fibrous material an alumina hydrate in the form of an aqueous dispersion; the alumina hydrate being as recited in any of claims 1 and 11 to 16. The process according to claim 28, wherein a coating weight of the dispersion containing an alumina hydrate is within a range of from 0.5 to 60 g/m 2 in a dry state.

Inventors: KONDO YUJI, MIURA KYO, YOSHINO HITOSHI, KONDO, YUJI, MIURA, KYO, YOSHINO, HITOSHI, Kondo, Yuji, c/o Canon K. K., Miura, Kyo, c/o Canon K. K., Yoshino, Hitoshi, c/o Canon K. K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: B41M1
IPC codes: C09D17, D21H21, C01F7, D21H17, B41M5